The present application relates to immersion lithography in integrated circuit manufacturing.
An integrated circuit (xe2x80x9cICxe2x80x9d) integrates a large number of electronic circuit elements, including transistors. The circuit elements are manufactured and interconnected internally on a semiconductor substrate, e.g., on a single crystalline silicon wafer. During manufacturing, the wafers undergo cycles of film deposition and lithography. Film deposition is the process of depositing a layer of material, e.g., insulating or metallic, over the entire substrate; lithography is the process of patterning the deposited layer. The first step in lithography involves coating the wafer with photoresist that is sensitive to particular radiation, typically ultra-violet light. During the next stepxe2x80x94exposurexe2x80x94the substrate is exposed to a radiation pattern stored on a mask, also called a reticle. Radiation locally changes the physical or chemical properties of the photoresist, and the exposed (or unexposed) areas are selectively dissolved during a developing step that leaves behind a pattern of photoresist. The patterned photoresist provides a pattern for a subsequent etching step. The etching step removes undesired areas of the deposited layer, leaving behind circuit elements, such as wires, resistors and transistors.
Highly integrated circuits require small circuit elements. Since the radiation pattern shapes the circuit elements, the smallest feature size depends on the resolution achieved in the lithography exposure step, or the resolution of the projection device used to project the radiation pattern onto the substrate. According to the Raleigh criterion, this resolution is proportional to the wavelength xcex of the projected light and to an adjustment factor k1, and inversely proportional to the sine function of the marginal, or capture, angle xcex8 of the projection optics:
Resolution=k1*xcex/sin(xcex8).xe2x80x83xe2x80x83(1)
The resolution can be decreased, i.e., improved, in one of three ways. First, the wavelength xcex of the projected light can be decreased. A shorter wavelength, however, may require new photoresist and a number of changes in the projection device, such as using a different light source and light filters, and special lenses for the projection optics. Second, the resolution can be decreased by decreasing the adjustment factor k1. Decreasing k1 may also require the use of different photoresist and high precision tools. Third, the marginal angle xcex8 can be increased by increasing the size of the projection optics. The effect of this increase, however, is limited by the sine function in Equation 1.